Many compositions and methods for chemical-mechanical polishing (CMP) the surface of a substrate are known in the art. Polishing compositions (also known as polishing slurries, CMP slurries, and CMP compositions) for polishing metal-containing surfaces of semiconductor substrates (e.g., integrated circuits) typically contain abrasives, various additive compounds, and the like, and frequently are used in combination with an oxidizing agent. Such CMP compositions are often designed for removal of specific substrate materials such as metals (e.g., tungsten or copper), insulators (e.g., silicon dioxide, such as plasma-enhanced tertraethylorthosilicate (PETEOS)-derived silica), and semiconductive materials (e.g., silicon or gallium arsenide).
In conventional CMP techniques, a substrate carrier (polishing head) is mounted on a carrier assembly and positioned in contact with a polishing pad in a CMP apparatus. The carrier assembly provides a controllable pressure (“down force”) to urge the substrate against the polishing pad. The pad and carrier, with its attached substrate, are moved relative to one another. The relative movement of the pad and substrate serves to abrade the surface of the substrate to remove a portion of the material from the substrate surface, thereby polishing the substrate. The polishing of the substrate surface typically is further aided by the chemical activity of the polishing composition (e.g., by oxidizing agents and other reactive chemicals present in the CMP composition) and/or the mechanical activity of an abrasive suspended in the polishing composition. Typical abrasive materials include, for example, silicon dioxide (silica), cerium oxide (ceria), aluminum oxide (alumina), zirconium oxide (zirconia), titanium dioxide (titania), and tin oxide.
The abrasive desirably is suspended in the CMP composition as a colloidal dispersion, which preferably is colloidally stable. The term “colloid” refers to the suspension of abrasive particles in the liquid carrier. The term “colloidal stability” and any grammatical variations thereof refers to the maintenance of a suspension of an abrasive particles during a selected period of time with minimal settling. In the context of this invention, an abrasive is considered colloidally stable if, when the suspension of abrasive is placed into a 100 mL graduated cylinder and allowed to stand without agitation for a period of time of 2 hours, the difference between the concentration of particles suspended in the bottom 50 mL of the graduated cylinder ([B] in terms of g/mL) and the concentration of particles suspended in the top 50 mL of the graduated cylinder ([T] in terms of g/mL) divided by the initial concentration of particles in the abrasive composition ([C] in terms of g/mL) is less than or equal to 0.5 (i.e., ([B]−[T])/[C]≦0.5). The value of ([B]−[T])/[C] desirably is less than or equal to 0.3, and preferably is less than or equal to 0.1.
U.S. Pat. No. 5,527,423 to Neville et al., for example, describes a method for chemically-mechanically polishing a metal layer by contacting the surface of the metal layer with a polishing slurry comprising high purity fine metal oxide particles suspended in an aqueous medium. Alternatively, the abrasive material may be incorporated into the polishing pad. U.S. Pat. No. 5,489,233 to Cook et al. discloses the use of polishing pads having a surface texture or pattern, and U.S. Pat. No. 5,958,794 to Bruxvoort et al. discloses a fixed abrasive polishing pad.
During polishing, the CMP slurry is continuously deposited onto the polishing pad, and excess slurry is continuously removed from the polishing pad at controlled rates of addition and removal. The CMP slurry on the polishing pad can accumulate waste materials (e.g., components of the substrate that are abraded or chemically etched from the surface of the substrate) during the polishing process, despite the fact that there is a continuous amount of slurry being removed from the pad. These waste materials can have a detrimental effect on the polishing process. For example, accumulation of metal ions, such as copper ions, can catalyze the decomposition of an oxidant, such as hydrogen peroxide, present in the CMP slurry. In addition, various reactive components in the CMP slurry, such as corrosion inhibitors and oxidizing agents, are consumed or degraded during the polishing process, which results in variations in the concentration of such reactive components during the course of polishing, compared to the concentrations in the fresh slurry. As the CMP process progresses, the differences in characteristics between the fresh and used slurry generally will vary over time, reflecting changes in the chemical reactions that are occurring on the substrate surface.
The polishing pad is also abraded and worn away during polishing, and must periodically be replaced when the wear on the pad becomes unacceptable. The CMP slurry present on the pad can accumulate abraded portions of the polishing pad. Often the extent of polishing pad wear, and the resulting decrease in polishing efficiency is not recognized until after a batch of semiconductor wafers has been removed from the polisher for further processing. Reworking or discarding unsatisfactorily polished wafers adds undesirable costs to the wafer manufacturing process.
There is an ongoing need to develop new CMP apparatus and methods for polishing substrates such as semiconductor wafers, in which the physical and chemical characteristics of the polishing slurry can be monitored in close proximity to the substrate during the polishing process, in real-time. The present invention provides such improved CMP apparatus and methods. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.